"sometimes the css works and sometimes not" looks like you it works for some url and doesn't work for some url. for this @rickz answer will work
href="${pageContext.request.contextPath}/css/adminStyle.css"
This will eliminate the possibility of breaking css import with regards of url
for second question "how do I access from a css file to an image in imgs folder"
background-image:url('../imgs/background.png');
because relative path of background image defined in css will start from the folder where your css file is.